Hollow turbine stator blades and rotor blades which are produced from superalloys, with film cooling holes out of which cooling air can flow during operation, are known. The cooling air creates a cooling film on the surface of the turbine blade, especially on its airfoil profile, which protects the turbine blade material against heat yield from the hot gas which flows around the turbine blade. These film cooling, holes penetrate the component wall and are arranged in the blade surface in such a way that the hole pattern, i.e. the arrangement of the holes in relation to each other and the hole geometry of each hole, and the cooling air mass flow which is blown through the holes, creates a continuous, protective cooling film.
On account of the particularly high thermal stresses, the regions of the turbine blades of the front turbine stages, which are exposed to hot gas, are additionally provided with anti-oxidation coatings and/or thermal barrier coatings. So-called MCrAlY coatings based on the corresponding superalloy, which are applied by plasma spraying processes, are used as anti-oxidation coatings. In addition, a thermal barrier coating of partially stabilized zirconium oxide is used. This oxide ceramic is applied after the MCrAlY coating by means of the electron beam physical vapor deposition process, or by means of atmospheric plasma spraying (APS), to the regions of the turbine blade which are to be protected.
The film cooling holes as a rule are introduced after the MCrAlY coating by means of laser drilling or spark erosion, especially if a thermal barrier coating is subsequently deposited by thermal evaporation by means of the EB-PVD process. If, instead, the APS process is used for the application of the thermal barrier coating, the introducing of the cylindrical holes is carried out by means of laser irradiation usually afterwards, that is through the ceramic thermal barrier coating. Since non-cylindrical, but profiled, diffuser-shaped film cooling holes are produced as profiled holes almost exclusively by means of spark erosion, these, however, must be introduced in advance because the ceramic thermal barrier coating is not electrically conductive and consequently production of the profiled holes is not correspondingly possible. If the profiled holes, however, are produced before the coating of the thermal barrier coating in the APS process, then these for the most part have to be very expensively protected by means of masking, because their geometry and contour would be unacceptably altered as a result of the coating (coat down effect).
Since the deposit of coating material in the profiled holes leads to an unacceptable impairment of the subsequent film cooling actions (on account of the consequently altered hole geometry, as a result of which the cooling air jet discharge width, the spatial direction of the cooling air impulse and the cooling air mass flow are altered), the coating material, subsequent to the coating (or subsequent to the recoating of a component which is to refurbished), has to be carefully removed from the covered profiled holes in a reworking process in order to reproduce the predetermined and desired geometry of the profiled holes for the need-based cooling and protective action of the cooling air which is blown out.
To this end, U.S. Pat. No. 6,380,512 B1 discloses a method for removing coating material from the discharge holes of cylindrical film cooling holes by means of laser irradiation. The largely program-controlled method determines the precise position of the hole which is to be made on the surface of the component, based on the electronically provided three-dimensional construction data, with reference to a defined holding system of the turbine blade. With a three-axis holding device for the turbine blade, which is associated with the device, this is moved by a CNC machine relative to a drilling laser and to a video optical system and positioned in space. On account of the geometry variations as result of operating stress and/or manufacturing and repair processes in the life cycle of the actual component, however, the actual geometry deviates from the theoretical or original three-dimensional definition. A video-camera with lenses orientated concentrically to the laser optics captures an image of the possibly closed discharge opening of the hole. The two-dimensional image which is projected in the longitudinal direction of the hole is then evaluated by means of image processing software, so that the position of the respective hole center point in the projection plane can be approximately determined from it. After the scanning of all the holes, these are compared with the 3D construction data in order to determine from it the most accurate actual position of the hole, i.e. with high probability. After that, the hole is exposed by means of laser drilling. The positioning of the turbine blade, the scanning of the possibly concealed hole position, the calculating of the probably actual position based on the 3D construction data, and the removing of the coating material, is then repeated for each hole.
Consequently, only a two-dimensional image of the component geometry ensues in a projection plane of the camera lens. These images have to be expensively linked to the three-dimensional construction data of the component, taking into account the associated component-dependent reference holding system, in order to be able to determine the actual position of the holes in the turbine blade. Therefore, the component must be held in a defined manner in a reference system in order to thus be able to make the connection between the three-dimensional component geometry and the position of the drilling laser in space via a common coordinate system.
A program-controlled processing of non-cylindrical hole structures or profiled holes (so-called fan-shaped holes) cannot be carried out by the described method from the closest prior art on account of the two-dimensional processing. Furthermore, the processing of the cylindrical holes, which to a great extent are partially closed off, or completely closed off, with coating material, with soldering material or with welding additive, depends upon the position identification software, wherein the known method, based on the stored data set, is limited with regard to the achievable positionability.
In particular, thermal barrier coatings which are applied in the APS process close off the cylindrical holes almost completely so that exposure of the holes which are at least partially closed off by it is not possible, or not possible with sufficient accuracy. Furthermore, three-dimensional structures in non-conducting ceramic coating, for example diffuser-shaped profiled holes, up to now cannot be processed by means of laser beam, and therefore in general cannot be mechanically processed subsequent to a cost-effective APS (re-)coating.
It is also known from U.S. Pat. No. 6,380,512 B1 that the determining of the actual position of each hole can be carried out exclusively by means of an optical scanning process, which is carried out before coating the turbine blade. However, this is very complex.